Civil Engineering Reference
In-Depth Information
TABLE 11-5. Upflow Clarifier Loading Rates for Water Temperatures
Greater Than 50
F (10
C)
Overflow Rate Based on
Total Clarifier Area
without Settling Tubes
gpm/ft
2
Overflow Rate Portion of
Basin Covered by
Settling Tubes
gpm/ft
2
Probable
Effluent
Turbidity (NTU)
m/h
m/h
2.0
4.9
2.0
4.9
1-3
2.0
4.9
3.0
7.3
1-5
2.0
4.9
4.0
9.8
3-7
2.5
6.1
2.5
6.1
3-7
2.5
6.1
3.0
7.3
5-10
Source:
Based on information from References 15 and 16.
Example of Applications
Horizontal Basins.
Suppose an existing water treatment plant with a rated capacity of
4 mgd (15 ML / d) has a horizontal, rectangular settling basin and rapid sand filters.
The basin dimensions are 30 ft wide by 133 ft long (9.1 m by 40.5 m). The surface
overflow rate at design capacity is 1,000 gpd / ft
2
(1.7 m / h). The average depth of the
basin is 15 ft (4.6 m). The basin has a single overflow weir across its outlet end. The
raw water is obtained from a river that has a normal maximum turbidity of 25 to 30
NTU. The water temperature rarely falls below 50
F (10
C). The settling basin is
preceded by mechanical flocculation with 40 min of detention time at 4 mgd (15 ML/
d). Coagulant aids are fed during periods of high turbidity and low water temperature
to improve coagulation.
A capacity increase from 4 to 8 mgd (15 to 30 ML / d) is desired. At 8 mgd, the
overflow rate increases to 2,000 gpd / ft
2
, or 1.4 gpm / ft
2
(3.4 m / h). The total basin
loading of 1.4 gpm / ft
2
is below any of the values shown in Table 11-7's guidelines.
However, at a higher basin loading of 2 gpm / ft
2
(4.9 m / h) and a tube rate of 3 gpm /
TABLE 11-6. Loading Rates for Horizontal-Flow Basins Where Water Temperatures Are
Frequently 40
F (4.4
C) or Less
Overflow Rate Based on
Total Clarifier Area
without Settling Tubes
gpm/ft
2
Overflow Rate Portion of
Basin Covered by
Settling Tubes
gpm/ft
2
Probable Effluent
Turbidity (NTU)
m/h
m/h
Raw-Water Turbidity
0-100 NTU
2.0
4.9
2.5
6.1
1-5
2.0
4.9
3.0
7.3
3-7
3.0
7.3
4.0
9.8
5-10
Raw-Water Turbidity
100-1,000 NTU
2.0
4.9
2.5
6.1
3-7
2.0
4.9
3.0
7.3
5-10
Source:
Based on information from References 15 and 16.
